Transcript Object-Oriented - University of Delaware

Object-Oriented Programming
James Atlas
June 12, 2008
1
Questions?
• Review…
 Java is …?
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Java Editions
• Different Java Editions
 SE: Standard Edition
 EE: Enterprise Edition
• Server-side applications
• Web applications, Communication (mail)
 ME: Micro Edition
• For PDAs, mobile devices, etc.
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Your Surveys
• Web application programming
Browser
(Client)
Web Application
Server
Servlets,
JSPs
Applet
 This class: briefly on client-side (Applets) and serverside programming (Servlets, JSPs)
 Good starting point to learn more on your own
 CISC474: Advanced Web Technologies
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Course Topics
• Java Fundamentals
• APIs
 GUIs
 Threads
 XML
 Network programming
 RMI
 Web applications: Servlets, JSPs
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A Little More Arrays
• Arrays is a class in java.util
• Methods for sorting, searching,
“deepEquals”, fill arrays
 Wouldn’t these methods have been useful in 105
or 181?
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Eclipse
• Everyone has it setup?
• Open project, find source code for Java SDK
• Let’s look at the java.util.Arrays class
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Today
• Object-Oriented Programming
 Objects
 Classes
 Instances
 Encapsulation
 Methods
 Inheritance
 Abstraction
 Polymorphism
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Object-Oriented Programming
• Programming that models real life
 Consider an ATM…
• Implicitly agreed upon interface between user and
the calculator
• What, not how
 Objects each have own role/responsibility
• As opposed to functional programming
 A list of instructions to the computer
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Objects
• How object does something doesn’t matter
• What object does matters (its functionality)
 What object exposes to other objects
 Referred to as “black-box programming”
Object
• Has public interface that others can use
• Hides state from others
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Objects: Black-box programming
• If an object allows you to access and store
data, you don’t care if the underlying data
type is an array or hashtable, etc.
 It just has to work!
 And, you don’t need to implement a new object
to do the job.
• Similarly, if object sorts, does not matter if
uses merge or quick sort
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Classes & Objects
• Classes define template from which objects
are made
 “cookie cutters”
 Define state - data, usually private
 Define behavior - methods for an object, usually
public
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Java Objects
• Java is pure object-oriented programming
 all data and methods in a program must be
contained within an object/class
 all classes are actually objects - instances of
java.lang.Class
• Example: java.lang.Object
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Instances
• Many objects can be created for a class
 Object: the cookie!
 Object, a.k.a. an instance of the class
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Instances Example
• Example: Chicken class
• State
 Weight, height, name
Chicken
Fred: 2.0lb,38cm
Sallie Mae: 3.0lb,45cm
Momma: 6.0lb,83cm
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Encapsulation Terminology
• Encapsulation is the combining of data and
behavior (functionality) into one package (the
object) and hiding the implementation of the
data from the user of the object.
• instance fields: the object’s data/variables
• methods: the functions & procedures which
operate on the object’s data
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Example: Chicken class
• State
 Weight, height, name
• Behavior
 Accessor methods
• getWeight, getHeight, getName
• Convention: “get” for “getter” methods
 Mutator methods
• eat: adds weight
• sick: lose weight
• changeName
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Exercise: Designing the Pizza Class
• State
• Behavior
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Constructors
• Constructor: a special method which
constructs and initializes an object
 After construction, you can call methods on an
object
• Constructors have the same name as their
classes
• To create an object of a certain type (a
certain class), use the new operator, much
like in C++
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Constructing objects using new
• Create three chickens
 “Fred”, weight: 2.0, height: 38
 “Sallie Mae”, weight: 3.0, height: 45
 “Momma”, weight: 6.0, height: 83
• Chicken constructor
 Chicken( String name, int height, double weight)
• Create new objects (Chickens)
 Chicken one = new Chicken(“Fred”, 38, 2.0);
 Chicken two = new Chicken(“Sallie Mae”, 45,
3.0);
And Momma? …
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Object References
• Variable of type object: value is memory
location
Chicken
weight =
2.0
height =
38
one =
two =
name = “Fred”
Chicken
weight =
3.0
height =
45
name = “Sallie Mae”
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Object References
• Variable of type object: value is memory
location
one =
two =
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If I haven’t called the constructor, only
Chicken one;
Chicken two;
Both one and two are equal to null
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Null Object Variables
• An object variable can be explicitly set to
null.
 indicates that this object variable does not
currently refer to any object.
• It is possible to test if an object variable is set
to null
Chicken chick = null;
… … …
if (chick == null)
{
. . .
}
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Exercise: Designing the Pizza Class
• Constructors?
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Relationship to C++
• Java object variables similar to object
•
pointers in C++
For example,
String myString;
// Java
is really the same as:
String * my_string;
// C++
• Every object variable in Java is like a
pointer to an object in C++
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Multiple Object Variables
• more than one object variable can refer to
the same object
Chicken sal = new Chicken(“Sallie Mae”);
Chicken sal2 = sal;
Chicken
sal =
sal2 =
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weight =
3.0
height =
45
name = “Sallie Mae”
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What happens here?
Chicken x, y;
Chicken z = new Chicken(“baby”, 1.0,
5);
x = new Chicken(“ed”, 10.3, 81);
y = new Chicken(“mo”, 6.2, 63);
Chicken temp = x;
x = y;
y = temp;
z = x;
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Encapsulation Revisited
• Objects should hide their data and only allow
other objects to access this data through
accessor and mutator methods.
• Common programmer mistake:
 creating an accessor method that returns a
reference to a mutable (changeable) object.
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Immutable Objects: java.lang.String
• Immutable objects have only accessors
• java.lang.String holds a char[]
• No method returns a reference to this char[]
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What is “bad” about this class?
class Farm
{
. . .
private Chicken headRooster;
public Chicken getHeadRooster()
{
return headRooster;
}
. . .
}
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Fixing the Problem: Cloning
class Farm
{
. . .
private Chicken headRooster;
public Chicken getHeadRooster()
{
return (Chicken) headRooster.clone();
}
. . .
}
Another Chicken object, with the same data as headRooster is
created and returned to the user.
If the user modifies (e.g., feeds) that object, it does not affect
headRooster.
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Cloning
• Cloning is a more complicated topic than it
seems from the example.
• We will examine cloning in more detail at a
later point.
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Access Modifiers
• A public method (or instance field) means
that any object of any class can directly
access the method (or field)
 Least restrictive
• A private method (or instance field) means
that any object of the same class can directly
access this method (or field)
 Most restrictive
• There are also other access modifiers, which
will be discussed with Inheritance
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Method Parameters in C
• Call-by-value – a copy of the parameter is
passed into the function/method.
• Call-by-reference – the memory location of
the parameter is passed into the
function/method.
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Method Parameters in Java
• Java always passes parameters into
methods call-by-value.
 methods cannot change the variables used as
input parameters.
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What’s the output?
int x;
x = 27;
System.out.println(x);
doubleValue(x);
System.out.println(x);
. . .
void doubleValue(int p)
{
p = p * 2;
}
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What’s the output?
Farm farm = new Farm(“OldMac”);
Chicken sal = new Chicken(“Sallie Mae”, 50, 10);
System.out.println(sal.getWeight());
farm.feedChicken(sal);
System.out.println(sal.getWeight());
. . .
void feedChicken(Chicken c)
{
c.setWeight( c.getWeight() + .5);
}
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What’s the difference?
• sal is an object variable, not an object
 refers to an object (think of it as being a pointer
to an object of the Chicken class).
• a copy of the object variable (the reference)
is passed into the method.
 a copy of a reference refers to the same object!
 we can modify the state of an object in this
manner
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Summary of Method Parameters
• Everything is passed call-by-value in Java.
• An object variable (not an object) is passed
into a method
 changing the state of an object in a method
changes the state of that object outside the
method
 the method does not see a copy of the original
object
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Varargs
• New in Java 1.5
• A method can take a variable number of
arguments
void varargsMethod(String ... args)
{
for (int i=0;i < args.length; i++)
System.out.println(“arg ”+ i +
“ is “ + args[i]);
}
Why would you want to do this?
Hint: Java now has a System.out.printf function
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Constructors
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Constructor Conventions
• A constructor is a special method that
•
•
•
•
•
defines an object’s initial state
Always has the same name as the class
A class can have more than one constructor
A constructor can have zero, one, or multiple
parameters
A constructor has no return value
A constructor is always called with the new
operator.
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Constructor Overloading
• Allowing more than one constructor (or any
method with the same name) is called
overloading.
 each of the methods that have the same name
must have different parameters
• Overload resolution is the process of
matching a method call to the correct method
by matching the parameters
 handled by the compiler
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Default Initialization
• Unlike C and C++, if an instance field is not
set explicitly in a constructor, it is
automatically set to a default value
 Numbers are set to zero
 Booleans are set to false
 Object variables are set to null
• It is a bad idea to rely on defaults
 Code is harder to understand
 Set all instance fields in the constructor(s).
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Default Constructor
• A default constructor is a constructor that has
no parameters.
• If no constructors are present in a class, a
default constructor is provided by the
compiler
 default sets all instance fields to their default
values
• If a class supplies at least one constructor
but no default constructor, the default
constructor is NOT provided
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Default Constructor
• Chicken class has only one constructor:
Chicken(String name, float weight, float height)
• So, there is no default constructor
Chicken chicken = new Chicken();
• Is a syntax error
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Explicit Field Initialization
• If more than one constructor needs to set a
certain instance field to the same value, the
initial state of the field can be set explicitly, in
the field declaration
class Chicken {
private String name = “”;
. . .
}
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Explicit Field Initialization
• Or in a static method call
class Employee
{
private int id = assignID();
. . .
private static int assignID()
{
int r = nextID;
nextID++;
return r;
}
}
More on static later…
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Explicit Field Initialization
• The ordering of explicit field initialization with
relation to the constructor is important.
• Explicit field initialization happens before any
constructor runs.
• if one constructor wants to change an
instance field that was set explicitly, it can.
• If the constructor does not set the field
explicitly, explicit field initialization is used
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final keyword
• An instance field can be made final.
• If an instance field is final, it must be set in
the constructor or in the field declaration and
cannot be changed after the object is
constructed
private final String type = Cards.HEARTS;
private final String id;
…
public MyObject( String id ) {
this.id = id;
}
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Constructors calling constructors
• It is possible to call a constructor from inside
another constructor.
• If the first statement of a constructor has the
form
this( . . . );
the constructor calls another constructor of
the same class.
 this keyword works just like in C++; an implicit
parameter
 refers to the object being constructed
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Constructors calling constructors
• Why would you want to call another
constructor?
 Reduce code size/reduce duplicate code
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Object Destructors
• In C++ (and many other OOP languages),
classes have explicit destructor methods that
run when an object is no longer used.
• Java does not support destructors, as it
provides automatic garbage collection
 Watches/waits until there are no references to
an object
 Reclaims the memory allocated for the object
that is no longer used
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finalize()
• Java supports a method named finalize().
• method is called before the garbage collector
sweeps away the object and reclaims the
memory
• This method should not be used for
reclaiming any resources
 the timing when this method is called is not
deterministic or consistent
 only know it will run sometime before garbage
collection
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Using finalize()
• What do we do in a finalize() method?
• We clean up anything that cannot be
atomically cleaned up by the garbage
collector
 Close file handles
 Close network connections
 Close database connections
 etc…
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Static Methods/Fields
• For related functionality/data that isn’t
specific to any particular object
• java.lang.Math
 No constructor (what does that mean?)
 Static fields: PI, E
 Static methods:
• static double ceil(double a)
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Static Methods
• Do not operate on objects
• Cannot access instance fields of their class.
• Can access static fields of their class.
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Static Fields
• A static field implies that there is only one
such field per class (not object!).
• All objects of a class with a static field share
one copy of that field.
• For example, if we wanted to have a unique
studentID for a Student class…
class Student {
private int id;
private static int nextID = 1;
. . .
}
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Static Fields
class Student {
private int id;
private static int nextID = 1;
. . .
}
• Each Student object has an id field, but
there is only one nextID field, shared
among all instances of the class
 nextID field is present even when no Students
have been constructed.
How would we use the nextID field to create unique IDs?
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Final Static Fields
• We can also use a static field to make a
constant in a class…
public class Math {
. . .
public static final
double PI = 3.14..;
}
• The Math class has a static constant, PI
 The value can be accessed using the Math
class: a = b * Math.PI;
• Notice we do not need to create an object of
the Math class to use this constant.
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main()
• The most popular static method we have
seen so far is the main() method.
• The main() method does not operate on
any objects
 It runs when a program starts…there are no
objects yet.
• main() executes and constructs the objects
the program needs and will use.
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Analyzing java.lang.String
• Look at Java Docs for Constructors
• String toUpperCase()
 Converts all of the characters in this String to
upper case
• static String valueOf(boolean b)
 Returns the string representation of the boolean
argument.
What methods/fields should be static for the Pizza class?
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Static Summary
• Static fields and methods are part of a class
and not an object
 do not require an object of their class to be
created in order to use them.
• When would we use a static method?
 When a method does not have to access an
object state (fields) because all needed data are
passed into the method.
 When a method only needs to access static
fields in the class
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(Returning) Explicit Field Initialization
• Or in a static method call
class Employee
{
private int id = assignID();
. . .
private static int assignID()
{
int r = nextID;
nextID++;
return r;
}
}
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Class Design/Organization
• Fields
 Chosen first
 Placed at the beginning or end of the class
 Has an accessor modifier, data type, variable
name and some optional other modifiers
• If no accessor modifer --> package-private
 Use this keyword to access the object
• Constructors
• Methods
 Maybe public static void main
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Next Class
• More Object-Oriented Programming
 Inheritance
 Abstraction
 Polymorphism
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Programming Assignment 1
• Available in MyCourses
• www.cis.udel.edu/~atlas/cisc370/assignments/assign1.html
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